1. Field of the Invention
This invention relates to a signal processing apparatus for receiving a signal that, when being sent out, has been amplified in a predetermined frequency component on the basis of a predetermined amplification characteristic and restoring the original signal.
2. Description of the Related Art
There has been known the recording and reproducing apparatus for recording image signals or other information signals on the magnetic recording medium such as a magnetic disk and reproducing them from the recording medium.
The conventional type of recording and reproducing apparatus is described by using FIG. 1 below.
In FIG. 1 there is shown an example of the apparatus for recording and reproducing luminance signals on and from a magnetic recording medium 9 by using a magnetic head 8. The luminance signal with inclusion of a synchronizing signal enters at an input terminal 1 and goes to a clamp circuit 2 where clamping occurs at the sync. tip portion or the like of the synchronizing signal for the luminance signal. In a non-linear emphasis circuit 3 and a linear emphasis circuit 4, emphasizing is applied to the luminance signal so that the S/N will be improved when the luminance signal is later reproduced. After having been modulated by a frequency modulator 5 and amplified by a recording amplifier 6, the luminance signal passes through a mode selector switch 7 to the magnetic head 8, by which it is recorded on the magnetic recording medium 9. In the reproduction mode, a minute reproduced signal output from the magnetic head 8 passes through the switch 7 to a pre-amplifier 10 where it is amplified to an adequate level. After having been demodulated by a frequency demodulator 11, a low-pass filter (LPF) 12 removes the superfluous signals. The thus-obtained signal appears at the output of the LPF 12.
Since the aforesaid LPF 12 has a group delay characteristic, it is in the neighborhood of its cutoff frequency fc that the group delay time G.sub.D rapidly increases (i.e., an overshoot takes place), as shown in FIG. 8(a). Therefore, an equalizer circuit 13 having an opposite group delay characteristic as shown in FIG. 8(b) to that shown in FIG. 8(a) of the LPF 12 is provided for making flat the apparent group delay characteristic by further processing the output of the LPF 12 through the equalizer circuit 13.
And, the output signal of the equalizer circuit 13 goes to a linear deemphasis circuit 14 having an opposite transfer function to that of the linear emphasis circuit 4 and then to a non-linear deemphasis circuit 15 having an opposite transfer function to that of the non-linear emphasis circuit 3. Thus, the luminance signal is reproduced, appearing at an output terminal 16.
FIG. 2 shows an example of the construction of the non-linear emphasis circuit 3. The luminance signal from its input terminal, on one hand, enters an adder 20 as it is, and, on the other hand, is routed to a high-pass filter (HPF) 17 where a high-frequency component is extracted out of the luminance signal. The thus-obtained high-frequency component of the luminance signal is compressed at a rate corresponding to the input level in compression circuit 18. After having been weighted by a coefficient multiplier circuit 19, it by now enters the adder 20, being added to the luminance signal being inputted from the input terminal. In such a manner, application of the non-linear emphasizing is performed. It should be noted that this non-linear emphasizing is characterized in that, as shown in FIG. 4, the emphasized quantity vary as different functions between when the level is high and when the level is low.
FIG. 3 exemplifies the construction of the emphasis circuit. To its input terminal, the luminance signal which was clamped at the time of, for example, the sync. tip portion of the synchronizing signal is applied. This input passes through a high-pass filter comprised of a capacitor C4 and a resistor R12 to an amplifier in the base-grounded form of a transistor Q5. To the collector of the transistor Q5, a soft limiter circuit is connected through a capacitor C5, comprising diodes D.sub.1 and D.sub.2 and resistors R16, R18, R19 and R20. The voltage (V)-current (I) characteristic of this soft limiter circuit is shown in FIG. 5. At the time of the low level, the characteristic is determined by r.sub.1 =R18=R19, while at the time of the high level, the characteristic is determined by r.sub.2 =R20=R16//R17. As a rule, r.sub.1 &gt;r.sub.2. This change of the characteristic is expressed by the formula: V.sub.D -(V.sub.2 -V.sub.1)/2, where V.sub.D represents the voltage across the turned-on diode D.sub.1, D.sub.2.
Here, if, as resistors R22 and R23 determine the value of (V.sub.2 -V.sub.1 ), R20=R22=2 is chosen, drift of the value VD of the diode D.sub.1, D.sub.2 can be canceled. With the help of a resistor R15, the gain of the amplifier is changed between the different values for the high and low levels by that soft limiter circuit. Thus, these parts constitute the compression circuit 18. The input signal subsequently goes through the emitter follower of a transistor Q4, a resistor R10 and a capacitor C3 to the emitter of a transistor Q3. Meanwhile, the luminance signal from the input terminal is applied through a resistor R9 to that transistor Q3 as a base-gounded type amplifier. The amplified signal passes through the emitter of a transistor Q2 to the linear emphasis circuit 4 comprised of a capacitor C1 and resistors R2 and R3 and therefrom through the emitter follower of a transistor Q1 to the output terminal.
By the way, a value of the coefficient K of the coefficient multiplier circuit 19 of FIG. 2 is determined, in the instance of FIG. 3, depending on the values of resistance of the R12, R15, r.sub.1, r.sub.2 and R10 and others.
FIG. 6 shows an example of the construction of the non-linear deemphasis circuit 15.
For this case, if the level at the input terminal of FIG. 2 and the level at the output terminal of FIG. 6 are equalized to each other, the transfer function of the compression circuit 18 becomes equal, so it can be said that, if the gain of the open loop of a differential amplifier 21 is sufficiently large, the transfer function of the non-linear deemphasis circuit 15 is equal to the reciprocal of the transfer function of the non-linear emphasis circuit 3.
FIG. 7 exemplifies the construction of the deemphasis circuit including the non-linear deemphasis circuit of FIG. 6. The reproduced luminance signal output from the frequency demodulator 11 is applied to the base of a transistor Q7, first passing through the linear deemphasis circuit which is formed with resistors R26 and R27 and a capacitor C7 and has the reverse characteristic to the linear emphasis characteristic, wherein non-linear de-emphasizing is applied by an amplifier of a transistor Q9. The thus-reproduced luminance signal appears at the emitter of a transistor Q10. To the emitter of the transistor Q10, a quite similar circuit of a high-pass filter and a compression circuit to the emphasis circuit of FIG. 3 is connected so that a signal component having the non-linear emphasis characteristic is produced at the emitter of the transistor Q10.
This signal is placed to the emitter of a transistor Q11 by an inverting amplifier of a transistor Q12, and therefrom applied through a resistor R35 to the emitter of a transistor Q9, thus forming a negative feedback loop for the non-linear emphasis section. By resistors R37, R38 and R35 and others, the value of the feedback coefficient becomes equal to the value of the coefficient K in the non-linear emphasis circuit.
By the way, in the apparatus of the character described above, to achieve a valuable increase of the resolution on the recording of the luminance signal, the carrier frequency of the frequency-modulated luminance signal to be recorded must be increased. But, because the electromagnetic conversion characteristics at the boundary between the magnetic head and the magnetic recording medium gets more deteriorated toward higher frequency bands, the edge portion of the reproduced signal has a conspicuous noise level as compared with the flat portion, contributing to an obstacle in the way of recording and reproducing video signals with a higher image quality.